1. Field of the Invention
The present invention relates to a piston for internal combustion engine having both fatigue strength and wear resistance.
2. Related Art
Conventionally, pistons for internal combustion engines (hereinafter, simply called as “piston”) are manufactured by a gravity casting method using an Al-Si metal alloy such as Japanese Industrial Standard JIS AC8A. In recent years, however, as engine output has been increased, fatigue strength and wear resistance are being improved, so that the content of additive elements such as Si, Cu, Ni and Mn tends to be increased. When 11 or more wt % of Si among such additive elements is added, it generates hypereutectic structures, and hard and granular primary crystal Si crystallizes so that the wear resistance is improved. For this reason, Si is widely adopted for Al alloys of pistons for high-power engines.
In these Al-rich Si alloys, however, coarse primary crystal Si crystallizes in casting, and the primary crystal Si causes fatigue breakdown so that the fatigue strength is deteriorated. Local uniformity of the structure occurs and hardness variation becomes large, so that the wear resistance in a softened portion is deteriorated and workability in a hardened portion is deteriorated.
In order to overcome the above disadvantages, for example in Japanese Patent No. 3043375 (in the section “Function”), when pistons are manufactured by the gravity casting method using an Al—Si alloy, P is added so that coarsening of primary crystal Si is suppressed. Furthermore, in Japanese Patent Application Laid-Open No. 10-219378 (in section 0008), after an Al alloy containing Si is dissolved and is rapidly solidified, a fine powder in which primary crystal Si is crushed is manufactured, and the powder is heat-extruded and molded so that a piston is manufactured.
In Japanese Patent Application Laid-Open No 2005-120891 (in the Abstract), after a piston is manufactured from an Al—Si alloy, in order to strengthen a necessary portion, a copper material is cast into that portion of the alloy by an electron beam.
The function of a piston is to form a firing pressure container, to contain the firing pressure, and to transmit the firing pressure. The pistons for internal combustion engines require different properties, depending on the portion, in order to fulfill this function. The typical required properties of the pistons include the following two properties:
(1) Wear resistance in sliding portions such as the top ring groove and the pin holing of a piston pin; and
(2) High fatigue strength in portions such as the surface of the piston combustion chamber, the pin boss rib, the outer peripheral surface of the lower portion of the pin boss and the skirt rib, on which a large stress load is applied by firing pressure and inertial force.
Unfortunately, the grain size of the primary crystal Si yields conflicting properties. That is to say, when the grain size of the primary crystal Si is large, the wear resistance is satisfactory, but the fatigue strength is deteriorated, and when the grain size of the primary crystal Si is small (or the primary Si is not present), the fatigue strength is high, but the wear resistance is deteriorated. In techniques for making the primary crystal Si fine as described in Japanese Patent No. 3043375 and Japanese Patent Application Laid-Open No. 10-219378, an entire portion of the piston has an approximately uniform structure, and thus the structures are not optimized for each portion. For this reason, in the pistons which require different properties depending on portions, one of the wear resistance and the fatigue strength is insufficient, and thus these techniques cannot sufficiently cope with high-power engines.
In the technique disclosed in the Japanese Patent No. 3043375 which makes the primary crystal Si fine by adding P, since the gravity casting method is used, the cooling rate at the time of the casting is relatively slow, and the mold temperature cannot be reduced in view of the molten metal. As a result, the primary crystal Si becomes coarse. For this reason, although the wear resistance of the top ring groove and the like can be secured by the primary crystal Si, the primary crystal Si crystallizes on the surface of the piston combustion chamber, the pin boss rib, the outer peripheral surface of a lower portion of the pin boss, and the casting surface of the skirt rib, and thus the fatigue strength of these portions is deteriorated.
In the technique disclosed in Japanese Patent Application Laid-Open No. 10-219378 which uses heat-extrusion and molding of powder so as to manufacture a piston, the grain size of the primary crystal Si can be reduced to an extent such that fatigue breakdown does not occur. Since, however, the wear resistance of the top ring groove and the like is insufficient, secondary particles such as SiC should be added to the powder. For this reason, the production costs of pistons are high.
In the technique disclosed in Japanese Patent Application Laid-Open No 2005-120891 which casts a copper material in parts so as to strengthen a piston, a measure for suppressing internal defects due to a gas generated at the time of melting the copper material, and a step of setting a distortion of the piston due to heating straightening is required. Since the number of steps including the melting step increases, the production cost becomes relatively high.